Ovens

ABSTRACT

A tunnel oven contains a plurality of transversely extending radiant heating elements 17, 18 arranged at intervals along the length of the oven 1, the oven 1 being provided with conveyor band 10 for transporting material to be heated through the oven radiant heating elements 17, 18 supplying thermal energy, a sensing tube 31, 32 which extends within the oven 1 over a part of the length of the oven 1 that contains at least two radiant heating elements 17 or 18 that are spaced from each other along the length of the oven 1, the sensing tube 31 or 32 being arranged to absorb radiation from the said at least two heating elements 17 or 18 with pump 34 for causing a liquid to flow through the sensing tube 31 or 32 at a controlled rate with thermocouple junctions 35, 36 for producing a signal that provides a measure of the temperature increase of the liquid resulting from its passage through the sensing tube 31, 32 within the oven, and control means 29, 30 and with processor 29 and valve controller 30 arranged to control the rate of emission of radiant heat by a radiant heating element or elements 17, 18 in accordance with the signal.

This invention relates to ovens of the kind that are suitable for use incontinuous processes in which material to be heated (which may be in theform of discrete articles) is transported through the oven and is heatedprogressively during its passage through the oven. Such ovens are knownas tunnel ovens because they are elongate and have at one end anentrance through which the material is introduced into the oven and, atthe other end, an exit through which the material is withdrawn. Tunnelovens are used for a variety of purposes, for example, to dry materialor to effect the baking of food products.

For many applications, it is important to control accurately the thermalconditions to which material is subject during its passage through atunnel oven. Accurate control is especially important when the oven isused to bake food products. Accordingly, although the invention is notlimited either to the use of tunnel ovens for the baking of foodproducts, for example, biscuits, or to tunnel ovens that are suitablefor that purpose, it is convenient to discuss the matter in terms of theoperation of a tunnel oven in a continuous process for the production ofdiscrete baked food products.

When a tunnel oven is used to effect the baking of food products, bakingproceeds progressively as the articles move through the oven. The lengthof time for which the articles are baked depends both on the path lengthof the articles within the oven and on the speed at which they areconveyed through it. The extent of the baking will, of course, alsodepend on the rate at which heat is transferred to the articles whilethey are in the oven.

It will be appreciated that, if a commercial baking process is to besuccessful, the baking conditions must be very carefully controlled. Notonly must the baking conditions for a given article be kept withinstrict limits, but there must be a high degree of uniformity in thebaking of all the articles of a given run.

The correct baking conditions for a particular kind of article havingbeen established within a tunnel oven, it might be supposed that itwould suffice simply to leave the settings of all the oven controlsunchanged, but that is not so. A variety of things can necessitatechanges in settings. For example, ovens are commonly provided withinspection windows at intervals along their length and, when aninspection window is opened, it will affect the conditions prevailingwithin the oven. A more serious problem arises when a run isinterrupted.

If for any reason it proves impossible to maintain the supply of unbakedarticles to the oven, or if the supply of unbaked articles has to bestopped because of a problem with the handling of the baked articlesleaving the oven, for example, a failure of packaging equipment, thenthe run is interrupted. If, while the run is interrupted, the settingsof the oven controls are left unchanged, the temperature within the ovenwill rise and, when the run is started again, the articles will beoverbaked until the conditions settle down. The overbaked articles willnot be acceptable, and there may be considerable wastage. The reason whystopping the supply of articles causes the temperature within the ovento rise is that the articles enter the oven at a relatively lowtemperature and leave it at a relatively high temperature, so that theyact as heat sinks.

The problems that arise when a run is interrupted manifest themselveseven more severely, but usually much less frequently, when an oven isstarted up after having been shut down, for example, for cleaning andmaintenance, and also when a run with one type of article comes to anend and a run with a different type of article is started. It will beappreciated that articles of different types will generally requiredifferent baking conditions and they may also have different coolingeffects.

With a view to avoiding problems of the kind referred to, many tunnelovens used for baking food products are provided with an automaticcontrol system, which is arranged to control the relevant settings inresponse to signals from one or more temperature sensors located withinthe oven.

In tunnel ovens, the articles may be heated in one or other of twobasically different ways: by radiation and by convection.

In what may be termed a radiation oven, radiant heating elements areprovided within the oven chamber, and the articles are heated primarilyby direct radiant heat transfer from the radiant heating elements to thearticles. The gas within the oven (which will usually be air having,when the radiant heating elements are burners, combustion products mixedwith it) will become hot, so that there will be some convective heating,and steps may be taken to promote that. Also, the walls of the oven willtend to become hot, and radiation emitted by the walls will alsocontribute to the heating of the articles. It is, however, only the rateof emission of radiation by the radiant heating elements that can becontrolled directly.

In what may be termed a convection oven, on the other hand, either gasis heated outside the oven and then supplied to it or it is heatedwithin the oven in a way that does not result in the incidence on thearticles of a significant flux of direct thermal radiation. The walls ofthe oven will tend to become hot, and radiation from the walls willcontribute to the heating of the articles, but that contribution willusually be small by comparison with the convective heating of thearticles by the hot gas. The heat required to produce the hot gas iscommonly, but not necessarily, produced by combustion and, when it is soproduced, the hot combustion products may themselves constitute at leasta part of the hot gas within the oven. Alternatively, where thecombustion takes place outside the oven, the hot combustion products maybe used to heat another gas, which is commonly air, by means of a heatexchanger. Only the other gas, and not the combustion products are thensupplied to the oven.

In a convection oven, if the relative velocity between the gas withinthe oven and the articles is maintained approximately constant, then thetemperature of the gas within the oven will provide a good measure ofthe net transfer of heat to the articles, and a control system that usesthat temperature as an input can be made to give reasonably accuratecontrol. In a radiation oven, on the other hand, convection heatingsupplies only a minor part of the heat transferred to the articles, evenwhen special steps are taken to promote convection heating. Accordingly,in a radiation oven, the temperature of the gas in the oven is a lesssatisfactory parameter on which to base control of the oven.

For purposes of research and development, measurements have been made ofthe net radiant "heat flux" incident on, and absorbed by, blackened testsurfaces located within a radiation oven. From such measurements thedegree of radiant heating of articles within radiation ovens can beestimated. Further, it has recently been suggested that means foreffecting such measurements, which include a small blackened coppersphere on which the radiation is incident, could be substituted for gastemperature sensors, which are commonly thermocouple junctions, incontrol systems for radiation ovens.

In a radiation oven, the radiant heating elements are usually ofelongate form and so mounted that they extend across the width of theoven at intervals along the length of the oven. The radiant heatingelements are commonly burners, to which a combustible fluid (usually, agas) and air are supplied. The combustible mixture emerging from aburner, once it has been ignited, burns to form a flame, and it iseither the flame or, where it is provided, a ceramic element heated bythe flame, from which heat is radiated to the articles. Other forms ofradiant heating element may, however, be used. Thus, for example, theradiant heating elements, or some of them, may take the form ofelectrical resistance heaters.

The main variable to be controlled in a radiation oven is the heatoutput of the radiant heating elements, including the possibility thatone or more elements will be cut out so that they supply no heat to theoven. Where the radiant heating elements are burners, the heat output ischanged by varying the rate of supply of combustible fluid to theburners (including reducing the rate of supply to zero). In that way,both the dimensions of the flames (and hence, at least in the absence ofceramic elements, the magnitude of the radiating areas), and thetemperature of the flames may be varied. With electrical resistanceheaters, the temperature of the elements is varied by changing themagnitude of the electric current through the elements. A control systemwill allow the heat output of the elements to be controlled individuallyand/or in blocks.

The invention provides a tunnel oven containing a plurality oftransversely extending radiant heating elements arranged at intervalsalong the length of the oven, the oven being provided with means fortransporting material to be heated through the oven, means for supplyingenergy to the radiant heating elements, a sensing tube which extendswithin the oven over a part of the length of the oven that contains atleast two radiant heating elements that are spaced from each other alongthe length of the oven, the sensing tube being arranged to absorbradiation from the said at least two heating elements, means for causinga liquid to flow through the sensing tube at a controlled rate, meansfor producing a signal that provides a measure of the temperatureincrease of the liquid resulting from its passage through the sensingtube within the oven, and control means arranged to control the rate ofemission of radiant heat by a radiant heating element or elements inaccordance with the signal.

When the oven, which is a radiation oven, is in operation, the rise intemperature of the liquid flowing through a sensing tube provides ameasure of the radiation flux that is incident on the tube, and hence ameasure of the radiation flux over at least a part of the length of theoven that is sufficiently great to contain at least two of the radiantheating elements. As is explained in more detail below, the signal thatprovides a measure of that temperature increase provides a betterparameter on which to base control of the oven than would not only asignal providing a measure of gas temperature within the oven, but alsoa signal providing a measure of the radiation flux at a point (strictly,over a small area not extending over a significant part of the length ofthe oven).

The radiant heating elements may be situated above the path along which,in use, the material is transported through the oven, the said sensingtube being then situated below the radiant heating elements and abovethe path along which, in use, material to be heated is transportedthrough the oven. Advantageously, there is provided an additional, lowerset of transversely extending radiant heating elements arranged atintervals along the length of the oven, the lower radiant heatingelements being situated below the path along which, in use, material tobe heated is transported through the oven, and wherein there is providedan additional, lower sensing tube situated above the lower radiantheating elements and below the path along which, in use, material to beheated is transported through the oven, and the lower sensing tubeextends over a part of the length of the oven that contains at least twolower radiant heating elements, the lower sensing tube being arranged toabsorb radiation from the said at least two lower radiant heatingelements, means for causing a liquid to flow through the sensing tube ata controlled rate, means for producing a signal that provides a measureof the temperature increase of the liquid resulting from its passagethrough the lower sensing tube within the oven, and control meansarranged to control the rate of emission of radiant heat by a lowerradiant heating element or lower radiant heating elements. The rises intemperature of the liquid flowing through the upper and lower sensingtubes are then determined mainly by the radiant heat output of the upperand lower radiant heating elements, respectively. Preferably, there areprovided a plurality of upper sensing tubes and a plurality of lowersensing tubes, the upper sensing tubes and the lower sensing tubes beingdistributed along the length of the oven, and there being provided meansfor causing a liquid to flow through all the sensing tubes, means forproducing a signal indicative of the temperature rise in the liquid ineach of the sensing tubes, and means for controlling the output of theradiant heating elements in accordance with the signals.

When there are provided a plurality of upper sensing tubes and aplurality of lower sensing tubes, then preferably the upper sensingtubes together extend over substantially the entire length of the oven,and the lower sensing tubes together extend over substantially theentire length of the oven.

Advantageously, when there are provided at least one upper sensing tubeand at least one lower sensing tube, the outer surface of the or eachupper sensing tube is radiation-absorbing above and radiation-reflectingbelow, and the outer surface of the or each lower sensing tube isradiation-absorbing below and radiation-reflecting above. Instead, theor each upper sensing tube may be provided with means arranged to shieldit against radiation that would be incident on it from below, and the oreach lower sensing tube may be provided with means arranged to shieldthe sensing tube against radiation that would be incident upon it fromabove. With either of those arrangements, the temperature rise in theliquid flowing through the or each upper sensing tube is determinedmainly by the radiant heat output of the upper radiant heating elements,but also partly by radiation from the roof of the oven (the top portionof the wall of the oven), and correspondingly for the or each lowersensing tube and the lower radiant heating elements.

It is usual to regard the oven as being made up of a number of zones,each extending over the entire transverse cross-sectional area of theoven and over a part of the length of the oven. The extent of any givenzone is determined by the fact that the power supply to all the radiantheating elements within the zone cannot be varied independently of oneanother, except that it may be possible to cut off entirely the powersupply to some only of the elements in a zone. When the oven can beregarded as being made up of such zones, it is preferable to provide atleast one upper sensing tube and at least one lower sensing tube in eachzone.

For many applications, especially the baking of food products, it isadvantageous that the or each sensing tube extends, as seen in plan, ina direction that is at an angle to the direction of movement of thematerial to be heated through the oven. If a sensing tube extendsparallel to the direction in which the material is transported throughthe oven, then the inevitable shielding of thermal radiation by asensing tube can result in a noticeably uneven heating of material thatpasses directly beneath an upper sensing tube or above a lower sensingtube. If, as is the case with the baking of food products in the form ofdiscrete articles, the material has a low thermal conductivity anduniform heating is important, it is especially desirable to reduce thelocalised shielding effect on any individual article by the or eachsensing tube.

The or each sensing tube may enter the oven at one side, extenddiagonally across the oven, and leave the oven at the other side.Advantageously, however, the or each sensing tube enters the oven at oneside, extends along a part of the length of the oven and across at leasta part of the width of the oven and returns substantially parallel toitself to leave the oven at the said one side. With the latterconfiguration of sensing tube, the increase in the temperature of theliquid flowing through the tube is more nearly a measure of theradiation flux incident on the sensing tube averaged (without weighting)over the length of the part of the oven over which the sensing tubeextends.

Advantageously, the means for producing a signal that provides a measureof the temperature increase of the liquid resulting from its passagethrough the or each sensing tube within the oven comprises thermocouplemeans.

Preferably, a plurality of sensing tubes is provided, each sensing tubebeing situated between (in a vertical direction) a group of adjacentradiant heating elements, and the path along which, in use, the materialis transported through the oven, so that each sensing tube is associatedwith a group of radiant heating elements, and the control means isarranged to control the rate of emission of radiant heat by the radiantheating elements of each group of radiant heating elements in responseto the signal that provides a measure of the temperature increase of theliquid that flows through the sensing tube with which those radiantheating elements are associated.

Advantageously, the means for transporting material through the oven isan endless band, the upper run of which extends through the oven andalong the length of the oven and which provides a supporting surface forthe material. The band may be imperforate, or it may be a mesh or beotherwise perforate.

It is usually preferable that the radiant heating elements are burners.Then, the energy that is supplied to the radiant heating elements is theenergy of combustion of the combustible mixture that is supplied to theburners.

The invention also provides a method of heating material, whichcomprises conveying the material through a tunnel oven in accordancewith the invention.

The method is especially useful when the material is a food material,and the heating effects baking of the material

Several forms of tunnel oven, each suitable for baking biscuits andconstructed in accordance with the invention, will now be described byway of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic side view of one form of oven, with somecomponents omitted in the interests of clarity;

FIG. 2 is a diagrammatic side view of a part of the oven shown in FIG.1, with part of the oven wall cut away and on a larger scale than FIG.1;

FIG. 3 is a transverse cross-section taken through the oven shown inFIGS. 1 and 2, and on a larger scale than FIG. 2;

FIG. 4 is a diagrammatic plan view showing the configuration of one ofthe sensor tubes shown in FIG. 3;

FIG. 5 is a diagrammatic plan view showing an alternative configurationfor a sensor tube;

FIG. 6 is a transverse cross-section, on an enlarged scale, takenthrough one of the sensor tubes shown in FIG. 3;

FIG. 7 is a side view, on the same scale as FIG. 6, of a part of anotherform of sensor tube assembly;

FIG. 8 is a cross-section taken on the line VIII--VIII of FIG. 7;

FIG. 9 is a cross-section taken on the line IX--IX of FIG. 7;

FIG. 10 is a side view, on the same scale as FIG. 7, of a part of afurther form of sensor tube assembly;

FIG. 11 is a cross-section taken on the line XI--XI of FIG. 10;

FIG. 12 is a cross-section taken on the line XII--XII of FIG. 10;

FIG. 13 is a side view, on the same scale as FIG. 10, of another part ofthe sensor tube assembly shown in FIG. 10;

FIG. 14 is a cross-section taken on the line XIV--XIV of FIG. 13;

FIG. 15 is a block diagram of the control means; and

FIG. 16 is a cross-section (not to scale) taken through two burner tubesand an associated convection-promoting element.

Referring to FIGS. 1, 2 and 3 of the accompanying drawings, the tunneloven comprises an elongate baking chamber, which is indicated generallyby the reference numeral 1. The chamber 1, which is open at each end, isbounded by a wall indicated generally by the reference numeral 2.

The oven wall 2 is (see FIG. 3) of double construction, comprising aninner skin 3 and an outer skin 4. The cavity between the inner skin 3and the outer skin 4 contains a thermally insulating material 5. Theoven wall 2 has a top portion (which constitutes the roof of the oven),two side portions and a bottom portion (which constitutes the floor ofthe oven), which are indicated generally (see FIG. 3) by the referencenumerals 6, 7a, 7b and 8, respectively. The oven wall 2 is supported bylegs 9 provided with feet 9a (see FIG. 3). In practice, the loads fromthe legs 9 are not fed directly into the wall 2 as shown in FIG. 3, butrather into a cradle (not shown) or other form of load-spreading andstrengthening means.

Extending through the oven chamber 1, and along its length, is the upperrun 10 of an endless conveyor band, which is indicated generally by thereference numeral 11. The lower, return, run 12 of the conveyor band 11passes beneath the bottom portion 8 of the oven wall 2, and henceoutside the oven chamber 1. Beyond the ends of the oven, the conveyorband 11 runs round a driven roller 13 at one end and an idler roller 14at the other end.

Between the driven roller 13 and the idler roller 14, the conveyor band11 runs (see FIG. 3) over support members 15a and 15b, which aresituated immediately beneath, and at intervals along the lengths of, theupper run 10 and the lower run 12 of the conveyor band, respectively.The upper support members 15a extend between the side portions 7a and 7bof the oven wall 2, while the lower support members each extend betweentwo oven legs 9 situated on opposite sides of the oven. Extendingupwardly from some of the support members 15a and 15b are two idlerrollers 16, which are mounted with their axes vertical and so as to befreely rotatable about their axes. The idler rollers 16 are so situatedtowards the ends of support members 15a and 15b that they are in contactwith the edges of the band 11. Thus, the idler rollers 16 on uppersupport members 15a serve to locate laterally the upper run 10 of theband 11, and the idler rollers 16 on lower support members 15b performthe same function for the lower run 12 of the band.

Extending transversely through the oven chamber 1 are upper and lowerradiant heating elements in the form of tubular burners, which areindicated by the reference numerals 17 and 18, respectively.

Each of the upper burners 17 consists of a tube which extends across thewidth of the oven chamber 1, is closed at one end, and has, extendingalong one side, a row of apertures. The apertures may, for example, beformed by having a slit that, within the oven chamber 1, extends alongthe length of the burner 17 on one side, and by providing within theslit a material, for example, expanded metal 17a (see FIG. 16), thatdivides the mouth of the slit into a plurality of discrete apertures.

Each upper burner 17 is inserted into the oven chamber 1 through anaperture in the side wall 7a (see FIG. 3), where the burner is mountedby means which is indicated generally by the reference numeral 19.Outside the oven chamber 1, the burner 17 is provided with an injectorand valve assembly which is shown schematically at 20 for effecting thesupply of a combustible gaseous mixture to the burner. The valve of theassembly 20 is used to shut off the supply of combustible gas to theinjector of the assembly and hence to the associated burner. Inoperation, compressed air is supplied to the injector where it entrainsthe combustible gas to form the combustible mixture that is fed to theburner 17. At the end remote from the assembly 20, the burner tube 17 isclosed and a portion of the burner 17 adjacent to that end is supported,for example, by a step in the side wall 7b of the oven, which step formsthe bottom of a vertically extending slot 21 formed in the side wall 7b.

The lower burners 18 are of the same form as, and are mounted in thesame way as, the upper burners 17. The lower burners 18 are alsoprovided with injector and control valve assemblies 20.

A compressor 21 for supplying air under pressure to the burners 17 and18 is mounted on top of the oven. The outlet from the compressor 21 isconnected to the injectors of the assemblies 20 associated with theburners 17 and 18 by a manifold of which a part only is shown at 22 (seeFIGS. 1 and 2).

The manifold 22 is provided with six control valves (not shown), ofwhich three control the rate of supply of compressed air to theinjectors of the assemblies 20 associated with the upper burners 17 ineach of the three zones (which zones are described below), and the otherthree control the rate of supply of compressed air to the injectors ofthe assemblies associated with the lower burners 18 in each of the threezones. Provided that the valve of an assembly 20 is open, so thatcombustible gas is supplied to the injector of the assembly, the rate atwhich a combustible gaseous mixture is supplied to the associated burner17 or 18 is determined by the rate of supply of compressed air to theinjector of the assembly. Thus, the rate of heat output of, say, theupper burners 17 in one zone, is determined (assuming that the supply ofcombustible gas to any one or more of them has not been shut off) by thesetting of the relevant control valve in the manifold 22.

Extending along the length of the oven is a plenum chamber 23, which isbounded by the roof 6 of the oven and a channel-shaped member 24 (seeFIG. 3). The plenum chamber 23 is in communication with the oven chamber1 through apertures 25 formed in the side walls of the channel-shapedmember 24 and arranged at intervals along the length of the plenumchamber. By means of two transversely extending partitions (not shown),the plenum chamber is divided into three compartments, and threeextractor fans 26, one for each compartment, are mounted on the top ofthe oven. The inlet to each of the extractor fans 26 is in communicationwith the associated compartment of the plenum chamber 23 by means of apipe 27, which passes through the top portion 6 of the oven wall 2.

The oven is provided with a control system. The main variables to becontrolled are the settings of the control valves in the manifold 22,which determine the rate of supply of compressed air to the injectors ofthe assemblies and hence the rate of supply of a gaseous combustiblemixture to the burners 17 and 18, and also the settings (open or shut)of the valves of the assemblies 20. A manual control is provided toenable the rate of extraction of gas from the oven chamber 1 to becontrolled by varying the settings of dampers provided at the inlets ofthe extractor fans 26. A manual control is also provided to enable thespeed at which the endless band 11 travels to be varied, for example, totake account of the different baking times required for differentproducts.

The control system (see FIG. 15) can be regarded as being made up ofthree parts: sensing means 28 for sensing the thermal conditionsprevailing within the oven chamber 1, processing means 29 for processingthe signals provided by the sensing means, and operating means 30, whichcontrols the rate of supply of combustible gaseous mixture to theburners 17 and 18.

For control purposes, the oven chamber 1 can be regarded as beingdivided into three zones, which are separated from one another byimaginary planes that each contain a different one of the central planesof the transverse partitions (not shown) in the plenum chamber 23. Thus,associated with each zone are the extractor fan 26 that extracts gasfrom the part of the plenum chamber 23 that is within the zone, and alsothe burners 17 and 18 that are within the zone.

As is explained hereinafter, the sensing means 28 is arranged to senseseparately the conditions that obtain within each of the three zones.Further the sensing means 28 is arranged to provide separate outputsignals for the region of each zone that lies above the upper run 10 ofthe band 11 and for the region of each zone that lies below the upperrun of the band. The sensing means 28 does not measure differences inthe thermal conditions between discrete points within each zone, butrather provides signals that are representative of the thermalconditions that obtain over the length (in the direction of travel ofthe band 11) of the upper and lower regions of the zone, that is to say,the regions above and below, respectively, the upper run 10 of the band.

In the interests of clarity, the sensing means 28 is not shown in FIG. 1but, referring to FIGS. 2 and 3, the sensing means for each zonecomprises an upper sensing tube 31 and a lower sensing tube 32. In eachzone, the upper sensing tube 31 is situated between the upper burners 17and the upper run 10 of the conveyor band 11, while the lower sensingtube 32 is situated between the upper run of the conveyor band and thelower burners 18. The sensing tubes 31 and 32 are made of stainlesssteel, and their outer surfaces are polished to enhance theirreflectivity. The portions of the outer surfaces of the tubes 31 and 32that lie above (in the case of the upper tubes 31) or below (in the caseof the lower tubes 32) the axes of the tubes are chemically blackened torender them good absorbers of radiation, as shown at 31a in FIG. 6.

The sensing tubes 31 and 32 each extend across the entire width of theoven chamber 1 and along substantially the entire length of the zone inwhich they are situated. The sensing tubes 31 and 32 each have the sameconfiguration when seen in plan, that configuration for one of the uppersensing tubes 31 being shown in FIG. 4. The positions of the upperburners 17 in the zone are indicated schematically by the lines 33.

Referring again to FIG. 4, a pump indicated schematically at 34 isarranged to pump a liquid through the upper sensing tube 31 (and alsothrough the adjacent lower sensing tube 32) at a constant rate, and twothermocouple junctions 35 and 36 are provided to sense the temperatureof the liquid in the sensing tube close to the inlet and outlet ends ofthe tube, respectively. The lower sensing tubes 32 are each providedwith thermocouple junctions (not shown) corresponding to thethermocouple junctions 35 and 36 of the upper sensing tubes 31.

The sensing means 28 thus consists of the upper and lower sensing tubes31 and 32, respectively, the pumps 34 and the thermocouple junctions 35and 36. The thermocouples containing the junctions 35 and 36 provide thesignals that are processed by the processing means 29. The operatingmeans 30, which acts in response to the signals after they have beenprocessed by the processing means 29, consists of the control valves inthe manifold 22, the valves of the assemblies 20, together withappropriate means for setting the control valves in response to theprocessed signals. The processing means 29 and the operating means 30may together be regarded as constituting control means for controllingthe rate at which radiant heat is emitted by the burners (or the flamesissuing from them) in response to the signals provided by the sensingmeans 28.

Referring to FIGS. 7, 8 and 9, each of the upper sensing tubes 31 may,instead of (or in addition to) being externally blackened over only theupper part of its outer surface, be provided with a hemi-cylindricalmetal shielding member 37, which has a reflective outer surface andwhich extends along the length of the underside of the tube. Atintervals along the length of the upper sensing tube 31, the shieldingmember 37 is held in position by a metal strap 38 which surrounds boththe sensing tube and the shielding member.

Although it is constructionally a little more complicated, it ispreferable if the shielding member is spaced from the sensing tube, andsuch an arrangement is shown in FIGS. 10, 11 and 12. In thatarrangement, a hemi-cylindrical shielding member 37a is spaced from thesensing tube 31, the radius of the inner surface of the shielding memberbeing larger than the radius of the outer surface of the sensing tube.At intervals along its length, the sensing tube 31 and the shieldingmember 37a are held together by a metal strap 38a surrounding both thesensing tube and the shielding member. At each strap 38a, apart-cylindrical, thermally insulating (preferably, ceramic) spacer 39is provided so that the shielding member 37a is there positively locatedwith respect to the sensing tube 31.

Each upper sensing tube 31 is held in position by suspending it from theroof 6 of the oven. Thus, as shown in FIGS. 13 and 14, some of the metalstraps 38a are surrounded by collars made up from hoops 40a which areclosed by plates 40b. Downward movement of the hoops 40a relative to theplates 40b is limited by nuts 40c, which are in screw-threadedengagement with end portions of the limbs of the hoops 40a. Welded tothe plates 40b are shanks 41 which are secured to the top portion 6 ofthe wall of the oven. It will be noted that the arrangement allows eachupper sensing tube 31 to undergo significant thermal expansion, such ascould occur if the flow of liquid through the tubes were to beaccidentally interrupted while the oven was in use.

While the sensing tube 31 shown in FIGS. 13 and 14 is provided with ashielding member 37a as shown in FIGS. 10, 11 and 12, the method ofsuspending the sensing tube 31 shown in FIGS. 13 and 14 can also be usedwhen the sensing tube is provided with a shielding member 37 as shown inFIGS. 7, 8 and 9, or when, as shown in FIG. 6, no shielding means isprovided.

Similarly, the lower sensing tubes 32, instead of (or in addition to)being externally blackened only below may also be provided withshielding members, which are supported from the bottom portion 8 of thewall 2 of the oven chamber 1. Those shielding members are arranged toshield the lower sensing tubes 32 from above. Thus, when they are viewedin transverse cross-section, the lower sensing tubes 32, and theassociated shielding means, are as shown in FIGS. 8 and 9 or as shown inFIGS. 11 and 12, but inverted. The arrangement for supporting each lowersensing tube 32 is similar to that shown in FIGS. 13 and 14, except thatthe shank is secured, not to the plate, but to the lowermost part of thehoop, and the shank extends downwardly to the floor 8 of the oven), andthe sensing tube rests on the hoop. In that arrangement, the nuts merelyserve to retain the plate (while leaving room for the lower sensing tube32 to expand).

In operation, a liquid is caused to flow through each upper sensing tube31 and each lower sensing tube 32 at a constant rate by the pump orpumps 34. Because the consequence of any leakage from the sensing tubes31 and 32 is, given that the material being heated consists of foodproducts, less serious if the liquid flowing through them is water, thatis the liquid used. The water is maintained under a pressure sufficientto ensure that it does not boil.

The temperature increase of the water as it flows through each of thesensing tubes 31 and 32 is measured, as by two thermocouples, onejunction of each thermocouple being shown at 35 and 36, respectively. Ifthe water were to boil during its passage through any of the sensingtubes 31 and 32, the measured temperature increase would be altered to asignificant, but indeterminate extent.

In a tunnel oven of this kind, where the radiant heating greatly exceedsthe convective heating, the heating of the sensing tubes 31 and 32results very largely from radiative heat transfer from the flamesissuing from the burners 17 and 18 and, to a lesser extent, fromradiation emitted by the roof 6 and the floor 8 of the oven,respectively. Further, because the upper sensing tubes 31 are reflectivebelow and/or shielded from below, while the lower sensing tubes 32 arereflective above and/or shielded from above, the temperature increase ofwater flowing through the upper sensing tubes 31 in a zone provides ameasure of the heat flux radiating from the flames issuing from theupper burners 17 and the roof 6 of the oven in the zone, while thetemperature increase of the water flowing through the lower sensingtubes 32 provides a measure of the radiant heat output from the lowerburners 18 and the floor 8 of the oven in the zone.

It is found that, even when the processing means 29 is of the kind usedwhen control of the oven is made dependent on measurements of the gastemperature within the oven, the replacement of means for measuring thegas temperature by the sensing means 28 as described above can greatlyimprove the performance of the oven. Further, as will now be explained,it is possible to arrange that such replacement can be made withouthaving to replace the entire control means.

When, as is now common, the control of the oven is determined by the gastemperature within the oven, the temperature is usually measured by athermocouple, and so the control means is arranged to receive as inputssignals from one or more thermocouples. In the case of the forms ofapparatus described above with reference to the accompanying drawings,it is possible to arrange that the temperature of the water enteringeach of the sensing tubes 31 and 32 is kept constant, when the onlymeasurement required is that provided by the thermocouple junction atthe downstream end of the sensing tube (for example, the thermocouplejunction 36). In that way, existing control systems can continue to beused if a thermocouple arranged to measure gas temperature within theoven is replaced by the sensing means 28 of the invention. Further, suchreplacement can significantly improve the performance of the oven,although some alteration of the control parameters will of course berequired.

It has also been found that the sensing means 28 in which the sensingtubes 31 and 32 of each zone extend over at least substantially thewhole length of the zone give significantly better results than areobtained if the existing means for measuring the gas temperature at onepoint (strictly, over a small area), or a few discrete points in thezone, are merely replaced by means for measuring the radiant heat fluxat such a point or points. That is because the upper sensing tube 31 inthe zone is subjected to radiation from all the upper burners 17 in thezone and from that part of the part of the roof 6 of the oven that iswithin the zone, whereas localised radiation sensors respond almostexclusively to the radiant heat sources that are in their immediatevicinity.

It has already been explained that the sensing means 28 may be varied invarious ways, for example, in the precise way in which the sensing tubes31 and 32 are shielded from below and above, respectively. The sensingmeans 28 may, however, also be modified in other ways.

It is clear that, with a sensing tube 31 having the configuration shownin FIG. 4, the temperature increase in water flowing through the tubewill not be affected equally by a given change in the output ofdifferent ones of the upper burners 17 in the zone. That is because thetemperature of the sensing tube 31 increases in the direction oppositeto the direction of movement of the upper run 10 of the band 11 (whichis in the direction shown by the arrow in each of FIGS. 1, 2, 4 and 5,that is to say, from left to right as seen in those FIGS.). Theresulting weighting of the measurement can be kept within limits thatwill commonly be found to be acceptable by ensuring that the temperatureof the water remains sufficiently low. Then, the difference intemperature between the flames from the upper burners 17 and the outersurface of the upper sensing tube 31 will be large by comparison withthe variation in temperature of the outer surface of the sensing tubealong its length. That is subject to the limitation, however, that thetemperature of the outer surface of the upper sensing tube 31 mustremain above the dew-point of the gas within the oven 1. Correspondingconsiderations apply to the lower sensing tubes 32.

It is also possible to reduce the effect of the variation in thetemperature of the outer surface of the upper sensing tube 31 along itslength by using a sensing tube of a different configuration. In FIG. 5,there is shown an upper sensing tube 31a of which the configuration issuch that the variation in the temperature of the tube along its lengthdoes not result in any significant weighting of the effect of thedifferent upper burners 17 beneath which it passes. In the case of theupper sensing tube 31a, the inlet and outlet ends of the tube aresituated close together and, instead of the direction of flow of thewater in the tube having over almost all the length of the tube acomponent antiparallel to the direction of movement of the upper run 10of the band 11, the direction of flow of the water through the tube has,over almost exactly one half of its length, a component antiparallel tothat direction and, over the other half of its length, a componentparallel to that direction. With that configuration of sensing tube, theaverage temperature of the water at adjacent points in the two limbs ofthe sensing tube 31a will vary only very little along the length of thezone, so that the weighting referred to above will be significantlyreduced.

It will be observed that (see FIG. 5) the sensing tube 31a extends overonly approximately one third of the width of the oven chamber 1. Thathas the advantage that it can be used in ovens where the presence ofobstructions within the oven chamber would prevent the fitting of asensing tube that extended further across the width of the oven. Otherthings being equal, however, it is desirable that the sensing tubesshould extend as far across the width of the oven as possible, for inthat way the angle between the direction in which the sensing tubeextends and the direction in which the articles are transported throughthe oven is made as large as possible. As a rough guide, an angle of 15°or more will generally be found to be satisfactory.

The sensing tubes, being at a relatively low temperature, have asignificant shielding effect on the radiation emanating from the burnersand, when the oven is used for the baking of food products, sensingtubes that extend parallel to (or very nearly parallel to) the directionof movement of the food products can result in the food products beingbaked noticeably less along lines where they have passed directly belowsensing tubes. The shielding effect of the sensing tubes is increasedwhen, as described above, they are provided with shielding means, andthen the angle of inclination of the tubes to the direction of travel ofthe articles needs to be larger if noticeably uneven baking is to beavoided.

The processing means 29 is arranged to receive and process separatelythe signals provided by the thermocouples 35 and 36 associated with eachof the sensing tubes 31 and 32, and so to cause the operating means 30to control the group of burners 17 or 18, respectively, that isassociated with one of the sensing tubes separately from the group ofburners associated with each of the other sensing tubes. The group ofupper burners 17 that is associated with a given upper sensing tube 31or 31a is made up of the burners that are located directly above thatupper sensing tube. Correspondingly, the group of lower burners 18 thatis associated with a given lower sensing tube 32 is made up of theburners that are located directly below that lower sensing tube. In thearrangement shown in FIG. 4, for example, there are six upper burners 17(of which the positions are indicated schematically by the lines 33)associated with a single upper sensing tube 31. Commonly, a zone willcontain from six to twenty upper burners 17, and from six to twentylower burners 18. The number of upper burners 17, and the number oflower burners 18 in a zone, may vary from zone to zone. Further, thenumber of lower burners 18 in a zone may differ from the number of upperburners 17 in the zone.

The operating means 30 operates the control valves in the manifold 22and the valves of the assemblies 20. Accordingly, the operating means 30can control the supply of a combustible gaseous mixture to all the upperburners 17 associated with a single upper sensing tube 31. Similarly,the operating means 30 can control the supply of a combustible gaseousmixture to all the lower burners 18 associated with a single lowersensing tube 32. In addition, the operating means 30 can cut offcompletely (and restore) the supply of a combustible gaseous mixture toindividual upper and lower burners 17 and 18, respectively.

The need to be able to cut off completely the supply of a combustiblegaseous mixture to individual burners 17 and 18 arises, at least inpart, because the rate of supply of gaseous mixture to a burner must notbe reduced so far that the velocity of the gaseous mixture issuing fromthe burner is less than the speed of travel of the flame front.Accordingly, it may be impossible in certain circumstances to reducesufficiently the total heat output of, say, all the upper burners 17 ina zone by operating the relevant control valve in the manifold 22. Whenthat total heat output has been reduced as far as possible by operationof the relevant control valve in the manifold 22, any further reductioncan be effected only by operating the valves of the assemblies 20associated with the burners 17 in question to cut off completely thesupply of combustible gas to one or more of the individual burners. If,as is common, flame detectors are provided, that will increase theminimum rate of supply of combustible gaseous mixture to the burners,because the flame detectors will indicate the absence of a flame atrates of supply of a combustible gaseous mixture that are more thansufficient to ensure that the velocity of the gaseous mixture issuingfrom the burners is significantly greater than the speed of travel ofthe flame front. Accordingly, when flame detectors are provided theprobability that the assemblies 20 associated with one or moreindividual burners 17 will have to be operated to cut off the supply ofcombustible gas to the burners 17 in question will be increased.

Although it is, for example, for the reason explained above, sometimesconvenient to use a thermocouple to measure the increases in thetemperature of the water as it flows through the sensing tubes 31 (or31a) and 32, platinum resistance thermometers may be used instead if theimproved accuracy resulting from their use is found to be necessary ordesirable.

As in known tunnel ovens of the kind intended for the baking of foodproducts, the band 11 may be imperforate (for example, a band of mildsteel) or it may be a mesh, depending largely on the nature of the itemsbeing baked. When the band 11 is a mesh, the lower burners 18 assume agreater importance because, instead of heating mainly the underside ofthe upper run 10 of the band, they heat the items being baked by directradiative transfer. Also, with a mesh, the need for good shielding(and/or high reflectivity) of the undersides of the upper sensing tubes31 and of the upper surfaces of the lower sensing tubes 32 is increased.

The oven itself may also be of a different construction. Thus, forexample, the oven chamber 1 may be supplemented by a lower chamberthrough which the lower run 12 of the band 11 returns. Then, if desired,there may be provided additional burners, similar to the lower burners18, arranged to preheat the underside of the lower run 12 of the band 11immediately before it leaves the supplementary chamber. Thus, the band11 is heated before it (that is to say, its upper run 10) receives theitems to be baked.

When such preheating burners are used, the sensing means 28 may includesensing tubes, situated between those burners and the lower run 12 ofthe band 11, the sensing tubes being similar to the lower sensing tubes32 (including having partial blackening or associated shielding).

The tunnel oven described above is to be regarded as a radiation ovenbecause of the presence within the oven of radiant heating elements inthe form of the burners 17 and 18. It does not cease to be a radiationoven if there are provided within the oven chamber 1 means forincreasing somewhat the element of convection heating. Such means may,for example, (see FIG. 16) comprise tubes 42, which extend betweenadjacent burners (upper burners 17 are shown), and are provided withrows of perforations 42a and 42b extending along their lower sides. Inthe interests of clarity, the tubes 42 and the burners 17 are shown on alarger scale than is the spacing between them. Gas is taken from theoven chamber 1 and recycled to the oven chamber via the tubes 42. Thegas supplied to the tubes issues from them in downwardly inclineddirections from the perforations 42a and 42b, and causes turbulence,which enhances the convective heating of the articles, although not tosuch an extent that it becomes comparable with the radiative heating.

Although, in the forms of apparatus described with reference to thedrawings, only one upper sensing tube 31 and only one lower sensing tube32 is provided for each zone, additional upper and/or lower sensingtubes may be provided if desired.

The apparatus described with reference to the accompanying drawings maybe varied in numerous other ways. For example, the burners 17 and 18 maybe replaced by so-called high efficiency burners, which include ceramicelements that are arranged to be heated by the flames issuing from theburners. The hot ceramic elements then become the principal source ofthermal radiation in the oven.

Also, referring to FIGS. 10, 11 and 12, the thermal insulation providedby the ceramic member 39 may instead be provided by metal studs or finsof small cross-sectional area provided on the shielding member 37a.Thus, a member (the ceramic member 39) having a large cross-sectionalarea, but made of a material having a low thermal conductivity, may bereplaced by members having a small total cross-sectional area but madeof metal, which has a high thermal conductivity.

It will be understood that, although the forms of tunnel oven describedwith reference to the accompanying drawings are baking ovens, theinvention is not limited to tunnel ovens intended for use as, and/orused as, baking ovens. Further, because accurate control of theconditions prevailing within the oven is usually more important when theoven is use for baking food products than it is for other applications,the control systems described will usually be satisfactory for otherapplications. In fact, for some other applications, it will be foundthat some of the features described, for example, the inclination of thesensing tubes with respect to the direction of travel of the material tobe heated, will not be necessary.

We claim:
 1. A tunnel oven containing a plurality of transverselyextending radiant heating elements arranged at intervals along thelength of the oven, the oven being provided with means for transportingmaterial to be heated through the oven, means for supplying energy tothe radiant heating elements, at least one sensing tube extending withinthe oven, the or each sensing tube extending over a part of the lengthof the oven that contains at least two radiant heating elements that arespaced from each other along the length of the oven, the or each sensingtube being arranged to absorb radiation from its associated at least twoheating elements, means for causing a liquid to flow through the or eachsensing tube at a controlled rate, means for producing a signal thatprovides a measure of the temperature increase of the liquid resultingfrom its passage through the or each sensing tube within the oven, andmeans arranged to control the rate of emission of radiant heat by aradiant heating element or elements in accordance with the signal.
 2. Atunnel oven as claimed in claim 1, wherein the radiant heating elementsare situated above the path along which, in use, the material istransported through the oven, and wherein the or each sensing tube issituated below the radiant heating elements and above the path alongwhich, in use, material to be heated is transported through the oven. 3.A tunnel oven as claimed in claim 2, wherein there is provided anadditional, lower set of transversely extending radiant heating elementsarranged at intervals along the length of the oven, the lower radiantheating elements being situated below the path along which, in use,material to be heated is transported through the oven, and wherein thereis provided at least one additional, lower sensing tube situated abovethe lower radiant heating elements and below the path along which, inuse, material to be heated is transported through the oven, and the oreach lower sensing tube extends over a part of the length of the oventhat contains at least two lower radiant heating elements, the or eachlower sensing tube being arranged to absorb radiation from itsassociated at least two lower radiant heating elements, means forcausing a liquid to flow through the or each sensing tube at acontrolled rate, means for producing a signal that provides a measure ofthe temperature increase of the liquid resulting from its passagethrough the or each lower sensing tube within the oven, and meansarranged to control the rate of emission of radiant heat by a lowerradiant heating element or lower radiant heating elements.
 4. A tunneloven as claimed in claim 3, wherein there are provided a plurality ofupper sensing tubes and a plurality of lower sensing tubes, the uppersensing tubes and the lower sensing tubes being distributed along thelength of the oven, and there being provided means for causing a liquidto flow through all the sensing tubes, means for producing a signalindicative of the temperature rise in the liquid in each of the sensingtubes, and means for controlling the output of the radiant heatingelements in accordance with the signals.
 5. A tunnel oven as claimed inclaim 4, wherein there are provided a plurality of upper sensing tubesand a plurality of lower sensing tubes, the upper sensing tubes togetherextend over substantially the entire length of the oven, and the lowersensing tubes together extend over substantially the entire length ofthe oven.
 6. A tunnel oven as claimed in claim 3, wherein the outersurface of the or each upper sensing tube is radiation-absorbing aboveand radiation-reflecting below, and the outer surface of the or eachlower sensing tube is radiation-absorbing below and radiation-reflectingabove.
 7. A tunnel oven as claimed in claim 3, wherein the or each uppersensing tube is provided with means arranged to shield it againstradiation that would be incident on it from below, and the or each lowersensing tube is provided with means arranged to shield the sensing tubeagainst radiation that would be incident upon it from above.
 8. A tunneloven as claimed in claim 1, wherein the or each sensing tube extends, asseen in plan, in a direction that is at an angle to the direction ofmovement of the material to be heated through the oven.
 9. A tunnel ovenas claimed in claim 8, wherein the or each sensing tube enters the ovenat one side, extends diagonally across the oven, and leaves the oven atthe other side.
 10. A tunnel oven as claimed in claim 8, wherein the oreach sensing tube enters the oven at one side, extends along a part ofthe length of the oven and across at least a part of the width of theoven, and returns substantially parallel to itself to leave the oven atthe said one side.
 11. A tunnel oven as claimed in claim 1, wherein themeans for producing a signal that provides a measure of the temperatureincrease of the liquid resulting from its passage through the or eachsensing tube within the oven comprises thermocouple means.
 12. A tunneloven as claimed in claim 1, wherein a plurality of sensing tubes isprovided, each sensing tube being situated between (in a verticaldirection) a group of adjacent radiant heating elements, and the pathalong which, in use, the material is transported through the oven, sothat each sensing tube is associated with a group of radiant heatingelements, and the control means is arranged to control the rate ofemission of radiant heat by the radiant heating elements of each groupof radiant heating elements in response to the signal that provides ameasure of the temperature increase of the liquid that flows through thesensing tube with which those radiant heating elements are associated.13. A tunnel oven as claimed in claim 1, wherein the means fortransporting material through the oven is an endless band, the upper runof which extends through the oven and along the length of the oven andwhich provides a supporting surface for the material.
 14. A tunnel ovenas claimed in claim 13, wherein the endless band is imperforate.
 15. Atunnel oven as claimed in claim 13, wherein the endless band is a meshor is otherwise perforate.
 16. A tunnel oven as claimed in claim 1,wherein the radiant heating elements are burners.
 17. A tunnel ovencontaining a plurality of transversely extending radiant heatingelements arranged at intervals along the length of the oven, the ovenbeing provided with means for transporting material to be heated throughthe oven, means for supplying energy to the radiant heating elements, asensing tube which extends within the oven over a part of the length ofthe oven that contains at least two radiant heating elements that arespaced from each other along the length of the oven, the sensing tubebeing arranged to absorb radiation from the said at least two heatingelements, means for causing a liquid to flow through the sensing tube ata controlled rate, means for producing a signal that provides a measureof the temperature increase of the liquid resulting from its passagethrough the sensing tube within the oven, and means arranged to controlthe rate of emission of radiant heat by a radiant heating element orelements in accordance with the signal.
 18. A tunnel oven as claimed inclaim 17, wherein the radiant heating elements are situated above thepath along which, in use, the material is transported through the oven,and wherein the said sensing tube is situated below the radiant heatingelements and above the path along which, in use, material to be heatedis transported through the oven.
 19. A tunnel oven as claimed in claim17, wherein there is an additional, lower set of transversely extendingradiant heating elements arranged at intervals along the length of theoven, the lower radiant heating elements being situated below the pathalong which, in use, material to be heated is transported through theoven, and wherein there is provided an additional, lower sensing tubesituated above the lower radiant heating elements and below the pathalong which, in use, material to be heated is transported through theoven, and the lower sensing tube extends over a part of the length ofthe oven that contains at least two lower radiant heating elements, thelower sensing tube being arranged to absorb radiation from the said atleast two lower radiant heating elements, means for causing a liquid toflow through the or each sensing tube at a controlled rate, means forproducing a signal that provides a measure of the temperature increaseof the liquid resulting from its passage through the lower sensing tubewithin the oven, and means arranged to control the rate of emission ofradiant heat by a lower radiant heating element or lower radiant heatingelements.
 20. A method of heating material, which comprises:conveyingthe material through a tunnel oven containing a plurality oftransversely extending radiant heating elements arranged at intervalsalong the length of the oven, the oven being provided with at least onesensing tube extending within the oven, the or each sensing tubeextending over a part of the length of the oven that contains at leasttwo radiant heating elements that are spaced from each other along thelength of the oven, the or each sensing tube being arranged to absorbradiation from at least two associated heating elements; supplyingenergy to the radiant heating elements; causing a liquid to flow throughthe or each sensing tube at a controlled rate; producing a signal thatprovides a measure of the temperature increase of the liquid resultingfrom its passage through the or each sensing tube within the oven; andcontrolling the rate of emission of radiant heat by a radiant heatingelement or elements in accordance with the signal.
 21. A method asclaimed in claim 20, wherein the material is a food material, and theheating effects baking of the material.